Method for reducing delay difference of differential transmission and system thereof

ABSTRACT

The present invention discloses a system and method for reducing delay difference of differential transmission, a certain delay difference between waveforms of the P signal and N signal is generated through controlling delay adjustment to P signal or N signal of the differential signals and controlling delay adjustment value simultaneously, to compensate for the delay difference of differential transmission due to the channels. Therefore, the present invention can reduce the delay difference of differential transmission due to property discrepancy of board materials and delay inconsistency among pins of the connectors, and at same time simplify the scheme design.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation of U.S. patent application Ser. No.11/519,283, filed Sep. 12, 2006, which claims priority to Chinese PatentApplication No. 200510102696.X, filed Sep. 13, 2005, both of which arehereby incorporated by reference in their entirety.

FIELD OF THE TECHNOLOGY

The present invention relates to signal transmission technology, andmore particularly, to a method and system for reducing delay differenceof differential transmission.

BACKGROUND OF THE INVENTION

Along with the rapid development of high speed interconnectiontechnology, the speed of channel transmission has become increasinglyhigher and has reached 10 Gbps. Generally, a pair of differentialsignals is adopted for high speed signal transmission. The pair ofdifferential signals includes two signals with opposite polarities, oneof which is defined as positive signal, i.e., P signal while the otheris defined as negative signal, i.e., N signal. Therefore, thedifferential signals should be transmitted on two channels, that is tosay, there are two channels for such signal transmission. At thereceiving end, the received signal is obtained through subtracting thepositive signal from the negative signal and then subsequent processessuch as level decision are applied to the obtained signal.

Ideally, the two differential signals with opposite polarities will betransmitted to a certain point of the channel between the transmittingend and the receiving end simultaneously. Therefore, the twodifferential signals will be transmitted to the detector of the chip atthe receiving end simultaneously. Accordingly, in order to ensure thereliability of signal transmission, the two signals with oppositepolarities should have the same delay during differential signaltransmission, as shown in FIG. 1, which means that the delay differenceof differential transmission should be zero, and only in this case, theoptimum received signal can be obtained through subtracting the positivesignal from the negative signal, as shown in FIG. 2.

However, due to certain reasons such as anisotropy of the printedcircuit board materials and discrepancy in real distance of differentialcable, the transmission delay of the two signals with oppositepolarities are inconsistent during the differential signal transmission,which means that the delay difference of the differential transmissionis not zero.

FIG. 3 is a schematic diagram of differential signals with non-zerodelay difference of differential transmission at the receiving end. FIG.4 shows a schematic diagram of the received signal obtained throughprocessing the two differential signals of FIG. 3. Obviously, the signalobtained through differential transmission shown in FIG. 4 is not anexpected signal.

Therefore, the bigger the delay difference of differential transmissionis, the heavier the signal jitters will occur at the receiving end, andthe larger the error of the optimum sampling points for Clock and DataRecovery (CDR) will become. Deterioration of the optimum sampling pointsmay further increase the Bit Error Ratio (BER) of received signal at thereceiving end, and deteriorate the system performance. As to the signalwith relatively high speed, any deterioration may result in a sharpincrease of BER, even to the extent of abnormal operation of system.

In order to solve the problem, a method for reducing delay difference ofdifferential transmission is put forward at present. The main idea ofthe method is implementing delay compensation to P signal and N signalat the receiving end. As shown in FIG. 5, the delay differencecompensation devices include two delay controllers, two delay modules, asubtracter, an error generating circuit and a threshold level decision(SLICER). The adaptive compensation for delay difference of differentialtransmission is to calculate the difference between the signals beforeand after the SLICER and obtain an error component. The device transmitsthe error component to the two delay controllers at the P end and the Nend, and the delay controllers further determine delay controlcomponents based on the error component. The delay modules determinedelay adjustment components at the P end and the N end according to thedelay control components, and further compensate the delay difference ofdifferential transmission generated in the channels.

SUMMARY OF THE INVENTION

The present invention provides a method for reducing delay difference ofdifferential transmission and a system thereof.

The system for reducing delay difference of differential transmissionincludes (1) a delay difference measuring device, measuring a delaydifference of differential signals received at receiving end; (2) adelay adjustment controlling device, determining a control informationaccording to the measured delay difference; (3) a delay adjustmentdevice, performing delay adjustment to the differential signalsaccording to the control information, wherein, when the delay differencebetween the second signal and the adjusted first signal is larger than apreset value, the first cross-point switch switches to transmit thesecond signal to the delay adjustment device for delay adjustment.

The method for reducing delay difference of differential transmissionincludes (1) measuring delay difference of differential signaltransmission at receiving end; (2) determining control informationaccording to the delay difference; (3) implementing delay adjustment tothe differential signals according to the control information; and (4)when the delay difference between the second signal and the adjustedfirst signal is larger than a preset value, the first cross-point switchswitching to transmit the second signal to the delay adjustment devicefor delay adjustment.

As seen from the above mentioned schemes, the system and method forreducing delay difference of differential transmission provided in thepresent invention is to generate a certain delay difference betweenwaveform of P signal and that of N signal through measuring the delaydifference of differential transmission and performing delay adjustmentto P signal or N signal of the differential signals based on themeasured delay difference, to compensate for the delay difference ofdifferential transmission generated in the channels. Therefore, thedelay difference of differential transmission caused by propertydiscrepancy of board materials and delay inconsistency among pins of theconnectors can be reduced, and at same time the scheme design issimplified.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic diagram of signals without delay differencereceived at the receiving end;

FIG. 2 is a schematic diagram of a signal obtained through performingsubtract operation to the signals without delay difference received atthe receiving end;

FIG. 3 is a schematic diagram of signals with delay difference receivedat the receiving end;

FIG. 4 is a schematic diagram of a signal obtained through performingsubtract operation to the signals with delay difference received at thereceiving end;

FIG. 5 is a schematic diagram of the device for removing delaydifference;

FIG. 6 is a schematic diagram of the system for reducing delaydifference of differential transmission in accordance with an embodimentof the present invention;

FIG. 7 is a schematic diagram of the delay adjustment controlling deviceof the system for reducing delay difference of differential transmissionin accordance with an embodiment of the present invention;

FIG. 8 is a schematic diagram of the delay difference measuring deviceof the system for reducing delay difference of differential transmissionin accordance with an embodiment of the present invention;

FIG. 9 is a schematic diagram of the delay adjustment device of thesystem for reducing delay difference of differential transmission inaccordance with an embodiment of the present invention;

FIG. 10 is a schematic diagram of the switching of crossbar switch inaccordance with an embodiment of the present invention; and

FIG. 11 is a schematic diagram of the delay difference measuring deviceof the system for reducing delay difference of differential transmissionin accordance with another embodiment of the present invention.

DETAILED DESCRIPTION OF THE INVENTION

The present invention is hereinafter described in detail with referenceto the accompanying drawings and the embodiments.

According to an embodiment of the present invention, the delaydifference between P signal and N signal of the differential signals atthe receiving end is measured, and then the P signal or N signal of thedifferential signals at the transmitting end is adjusted based on thedelay difference. The delay difference of differential transmission iseliminated or reduced through measurement, feedback and adjusting.

The delay difference measured at the receiving end may be voltagedifference or time difference.

The invention will be described in more detail hereinafter withreference to the embodiments by taking the measured voltage differenceas an example.

The system for reducing delay difference of differential transmissionaccording to the embodiment of the present invention includes, as shownin FIG. 6, a delay adjustment device, a delay adjustment controllingdevice, a delay difference measuring device and two crossbar switches.

The delay adjustment device is used for implementing delay adjustment tothe differential signals.

The input signals of the first crossbar switch are P signals or Nsignals, and one output signal of the first crossbar switch istransmitted to the second crossbar switch via the delay adjustmentdevice while the other output signal is transmitted to the secondcrossbar switch directly. Through the first crossbar switch, it ispossible to control whether to output the P signal or N signal to thedelay adjustment device, which facilitates the delay adjustment deviceto adjust the P signal or N signal and further eliminates the delaydifference of differential transmission.

The input signals of the second crossbar switch are the adjusteddifferential signals. The second crossbar switch can control the outputof two definite signals, i.e., it is definite that the component of theoutput is P signal or N signal. Obviously, in cases when the receiverdoes not care the component of the output is N signal or P signal, thesecond crossbar switch is not necessary. Alternatively, if the P signaland N signal of the differential signals can be identified through othermeans, the second crossbar switch is optional.

The delay difference measuring device is used for measuring the delaydifference of the differential signals received at the receiving end.The delay adjustment controlling device, based on the measured delaydifference, determines the control information for controlling the firstcrossbar switch and the delay adjustment device to implement delayadjustment. In this way, the delay adjustment device can adjust thedifferential signals based on the actually measured delay difference,thereby ensuring the reduction of the delay difference of differentialtransmission.

As shown in FIG. 7, the delay adjustment controlling device includes aregister group and a comparator.

The register group includes three registers, as follows.

The first register, for storing the current measured value of delaydifference.

The second register, for storing the last measured value of delaydifference.

The third register, for storing the last but one measured value of delaydifference.

The comparator is used for comparing the measured values stored in theabove three registers, obtaining control information and then outputtingthe control information to the delay adjustment controlling device.

As shown in FIG. 8, the delay difference measuring device includes anadd circuit, a DC blocking and absolute value integrating circuit, apeak detecting circuit and an integration clear circuit.

The add circuit is used for adding the received P signal with thereceive N signal, and then outputting the added signal to the DCblocking and absolute value integrating circuit.

The DC blocking and absolute value integrating circuit is used forblocking the DC component of the added signal and performing absolutevalue integrating calculation to the signal, and then outputting theresult to the peak detecting circuit.

The peak detecting circuit is used for determining the peak value of thesignal outputted by the DC blocking and absolute value integratingcircuit, and outputting the peak value to the delay adjustmentcontrolling device.

The integration clear circuit is used for clearing the DC blocking andabsolute value integrating circuit after each integrating calculation.

As shown in FIG. 9, the delay adjustment device includes a set of delayunits and a delay controlling switch.

All the delay units are cascaded together, and the input signal istransmit from the input of the first delay unit, while the output ofeach delay unit connects with the delay controlling switch. The delayadjustment component of each delay unit is expressed as τ.

The delay controlling switch determines the current delay adjustmentvalue based on the output of the register group, and selects the outputof corresponding delay unit as the output signal after delay adjustment.

In this embodiment, P signal or N signal of the differential signalswhich are to be sent needs to be transmitted to the delay adjustmentdevice through the first crossbar switch, while the other signals aretransmitted directly through the first crossbar switch, thereby reducingthe delay difference of differential transmission effectively andsimplifying the design of the chip.

In this embodiment, the transmitting end and receiving end adoptautomatic negotiation mode to adjust the delay difference ofdifferential transmission, which means continuing to measure the delaydifference and adjusting the delay of differential signals until thedelay difference of differential transmission is less than a presetvalue.

In the automatic negotiation mode, the transmitting end sends a stringof specific serial codes or clock data as test differential signals, andthe receiving end receives and processes the serial codes or clock data.The specific serial code may be the code with specific model. Forexample, there are two sub serial codes in each serial code, oneincludes N 1s and the other includes N 0s, e.g., 110011001100 or0011001100110011, wherein N is a positive integer. In order to make theautomatic negotiation easier, the specific serial code should be thecode on which the receiving end and transmitting end agree. Meanwhile,in order to reduce negative influence due to the channel high-frequencyloss and intersymbol interference on measuring precision of delaydifference at the receiving end, the signal transmission rate can belowered in the automatic negotiation mode. For instance, thetransmission rate of the test differential signal can be lowered to 1/10of the normal working rate of the chip.

In order to further reduce the negative influence due to channel lossand intersymbol interference on measuring precision of delay difference,the test differential signal can be sharpened or attenuated in advanceat the transmitting end, which makes the waveform of signal similar tothe channel's optimal transmission waveform. The channel's optimaltransmission waveform means that the frequency spectrum of the signaltransmitted in the channel is the reciprocal of the channelcharacteristics. In this way, the optimal waveform can be received atthe receiving end. All these techniques are helpful for improving themeasuring precision of delay difference of differential transmission.

After the automatic negotiation ends, i.e., the delay difference ofdifferential transmission is less than a preset value, the transmittingend starts to transmit actual differential signals.

In this embodiment, the delay adjustment device is located at thetransmitting end, while the delay adjustment controlling device and thedelay difference measuring device are located at the receiving end.Obviously, the delay adjustment device may be also located at thereceiving end.

The operating principle of each device of the system for reducing delaydifference of differential transmission will be described in detailhereinafter.

First, at the receiving end, the delay difference measuring devicemeasures the delay difference between the received P signal and Nsignal. The measuring process includes the following steps.

(1) Sum the P signal and N signal, and the obtained voltage component isthe common-mode voltage of the differential signals.

(2) Implement DC blocking and absolute value integration to thecommon-mode voltage.

For ideal differential signals, the common-mode voltage obtained bysumming P signal and N signal and DC blocking should be zero, and theresult of absolute value integration is also zero. For actualdifferential signals, however, the common-mode voltage is not zero dueto the delay difference of differential transmission, therefore, theoutput of the DC blocking and absolute value integrating circuit is notzero.

(3) Peak detection circuit detects the peak value of the signaloutputted by the DC block and absolute value integrating circuit, andoutputs the peak value to the delay adjustment controlling device.

Second, the delay adjustment controlling device obtains the delayadjustment controlling information based on the delay differencemeasured by the delay difference measuring device. The procedure furtherincludes the following steps.

(1) The delay adjustment controlling device receives the delaydifference measured value from the delay difference measuring device.

(2) Save the measured values of the delay difference received in lastthree times: the first register saves the current measured value; thesecond register saves the last measured value; while the third registersaves the last but one measured value.

(3) Compare the measured values of the delay difference saved in thethree registers, and obtain the delay adjustment controllinginformation.

{circle around (1)} if the current measured value saved in the firstregister is larger than the last measured value saved in the secondregister, and the last measured value is bigger than the last but onemeasured value saved in the third register, output a control informationof switching the crossbar switch.

{circle around (2)} if the current measured value saved in the firstregister is bigger than the last measured value saved in the secondregister, and the last measured value is less than the last but onemeasured value saved in the third register, output a control informationof ending the delay adjustment.

{circle around (3)} if the current measured value saved in the firstregister is less than the last measured value saved in the secondregister, output a control information of continuing to increase thedelay adjustment value.

Third, when the delay adjustment controlling device sends the controlinformation of switching the crossbar switch, as shown in FIG. 10, thetwo crossbar switches implement the following switching process:

The state 1 is P signal connects to the delay path while N signalconnects to the normal path. The state 2 is P signal connects to thenormal path while N signal connects to the delay path. The switchingprocess is just switching from state 2 to state 1, or switching fromstate 1 to state 2.

Fourth, when the delay adjustment controlling device sends the controlinformation of ending the delay adjustment, the delay adjustment isterminated.

Fifth, when the delay adjustment controlling device sends the controlinformation of continuing to increase delay adjustment value, the delayadjustment device chooses to add a delay adjustment component to theoriginal delay adjustment value without switching the crossbar switches,and then implements delay adjustment to the transmitting signalaccording to the reset delay adjustment value.

When the delay adjustment controlling device sends the controlinformation for starting delay adjustment, the delay adjustment devicechooses the minimum delay adjustment value, selecting the correspondingswitch to implement delay adjustment to the transmitting signal based onthe delay adjustment value.

In above mentioned process, the precision of the delay adjustment deviceis τ, which is the delay adjustment value of each delay unit of thedelay adjustment device. It is possible to reduce τ value to improve thedelay adjustment precision of the system.

According to the present embodiment, the time difference can be measuredand be adopted to act as the input parameter of the delay adjustmentcontrolling device. In this way, the delay difference measuring deviceis a time difference measuring device. As shown in FIG. 11, themeasuring procedure of the time difference measuring device includes thefollowing steps:

P signal and N signal are sent respectively to the edge triggeringcircuit, and the edge triggering circuit is responsible for initiatingand terminating the RC integrating circuit. When the P signal arrivesfirst, the edge triggering circuit initiates the RC integrating circuitto start integrating, and when the N signal arrives, the edge triggeringcircuit terminates the RC integrating circuit to stop integrating.Alternatively, when the N signal arrives first, the edge triggeringcircuit initiates the RC integrating circuit to start integrating, andwhen the P signal arrives, the edge triggering circuit terminates the RCintegrating circuit to stop integrating.

The voltage-time converting circuit converts the obtained integratingvoltage into time difference, according to following formula:

t=−RCln(1−vV);

wherein, t is the time difference; v is the measured voltage value; V isvoltage of power source of the RC integrating circuit; R is theresistance value and C is the capacitor value.

To sum up, a certain delay difference between waveforms of the P signaland N signal is generated through controlling the implementation ofdelay adjustment to P signal or N signal of the differential signals andcontrolling the delay component simultaneously, to compensate for thedelay difference of differential transmission occurring in the channels.Therefore, the present invention can solve the delay difference ofdifferential transmission due to property discrepancy of board materialsand delay inconsistency among pins of the connectors, and meanwhilesimplifies the scheme design.

The foregoing are only the preferred embodiments of the presentinvention and are not intended to limit the scope of the presentinvention. Any modification, equivalent substitution, or improvementmade without departing from the spirit and principle of the presentinvention should be covered by the scope set forth in the appendedclaims.

1. A system for reducing delay difference of differential transmissioncomprising: a first cross-point switch, adapted to receive a positivesignal and a negative signal of a differential signal, switch one of thepositive signal and the negative signal as a first signal to a delayadjustment device for delay adjustment, and switch the other one of thepositive signal and the negative signal as a second signal output; thedelay adjustment device, adapted to perform delay adjustment to thefirst signal to obtain an adjusted first signal; a delay adjustmentcontrolling device, adapted to obtain a delay difference between thesecond signal and the adjusted first signal, determine controlinformation according to the delay difference, and transmit the controlinformation to the delay adjustment device for controlling latter delayadjustment of the delay adjustment device, wherein, when the delaydifference between the second signal and the adjusted first signal islarger than a preset value, the first cross-point switch switches totransmit the second signal to the delay adjustment device for delayadjustment.
 2. The system according to claim 1, further comprising: asecond cross-point switch, adapted to receive the second signal and theadjusted first signal, and output the second signal and the adjustedfirst signal according to a positive and negative definition.
 3. Thesystem according to claim 1, wherein the delay adjustment controllingdevice comprises: a first register, adapted to store a current value ofthe delay difference; a second register, adapted to store a last valueof the delay difference; a third register, adapted to store a second tolast value of the delay difference; a comparator, adapted to determinethe control information through comparing the values of the delaydifference stored in the registers and output the control information tothe delay adjustment device.
 4. The system according to claim 3, whereinthe comparator is further adapted: to determine that the controlinformation being switching from adjusting the first signal to adjustingthe second signal if the current value is larger than the last value andthe last value is larger than the second to last value; to determinethat the control information being maintaining a current delayadjustment value if the current value is larger than the last value andthe last value is smaller than the second to last value; and todetermine that the control information being increasing delay adjustmentvalue if the current value is smaller than the last value.
 5. The systemaccording to claim 1, wherein the delay adjustment device comprises adelay controlling switch and at least one delay unit; wherein: the delaycontrolling switch has an output and at least two inputs, the number ofthe inputs of the delay controlling switch equals to the number of thedelay unit(s) plus one; one input of the delay controlling switch isadapted to receive the first signal, other input(s) of the delaycontrolling switch is(are) respectively connected with an output of eachof the at least one delay unit; the at least one delay unit is adaptedto receive the first signal and perform delay adjustment to the firstsignal; and the delay controlling switch is adapted to determine currentdelay adjustment value according to the control information outputtedfrom the delay adjustment controlling device, and select to put throughone of the inputs of the delay controlling switch to the output of thedelay controlling switch.
 6. The system according to claim 1, whereinthe delay adjustment device is located at a transmitting end of thedifferential signals.
 7. The system according to claim 1, wherein thedelay adjustment device is located at a receiving end of thedifferential signals.
 8. The system according to claim 1, furthercomprising: a delay difference measuring device, adapted to receive thesecond signal and the adjusted first signal, measure the delaydifference between the second signal and the adjusted first signal, andtransmit the delay difference to the delay adjustment controllingdevice.
 9. The system according to claim 8, wherein the delay differencemeasuring device comprises: an add circuit, adapted to receive thesecond signal and the adjusted first signal, sum the second signal andthe adjusted first signal to obtain a first result; a direct current(DC) blocking and absolute value integrating circuit, adapted to block aDC component of the first result outputted by the add circuit, andintegrate an absolute value of the first result to obtain a secondresult; a peak detecting circuit, adapted to determine a peak value ofthe second result outputted by the DC blocking and absolute valueintegrating circuit, and output the peak value as the delay differenceto the delay difference adjustment controlling device; and anintegration clear circuit, adapted to clear the DC blocking and absolutevalue integrating circuit after each integrating calculation.
 10. Thesystem according to claim 8, wherein the delay difference measuringdevice comprises an edge triggering circuit, a resistance capacitor (RC)circuit, a voltage-time converting circuit and an integration clearcircuit; the edge triggering circuit is adapted to receive the secondsignal and the adjusted first signal, and adapted to start and stop theRC integrating circuit in response to the second signal and the adjustedfirst signal received; the RC integrating circuit is adapted to startintegration and stop integration under control of the edge triggeringcircuit, and output an integrated voltage; the voltage-time convertingcircuit is adapted to convert the integrated voltage into a timedifference, and output the time difference as the delay difference tothe delay difference adjustment controlling device; and the integrationclear circuit is adapted to clear the RC integrating circuit after eachintegrating processing.
 11. A method for reducing delay difference ofdifferential transmission comprising: receiving, by a first cross-pointswitch, a positive signal and a negative signal of differential signals,switching one of the positive signal and the negative signal as a firstsignal to a delay adjustment device for delay adjustment, and switchingthe other one of the positive signal and the negative signal as a secondsignal output; performing, by the delay adjustment device, delayadjustment to the first signal to obtain an adjusted first signal;obtaining, by a delay adjustment controlling device, a delay differencebetween the second signal and the adjusted first signal, determiningcontrol information according to the delay difference, and transmittingthe control information to the delay adjustment device for controllinglatter delay adjustment of the delay adjustment device; and when thedelay difference between the second signal and the adjusted first signalis larger than a preset value, the first cross-point switch switching totransmit the second signal to the delay adjustment device for delayadjustment.
 12. The method according to claim 11, further comprising:summing the second signal and the adjusted first signal of thedifferential signals to obtain a first result; blocking a direct current(DC) component of the first result and then integrate an absolute valueof the first result to obtain a second result; and detecting a peakvalue of the second result, and taking the peak value as the delaydifference.
 13. The method according to claim 11, further comprising:integrating, by a resistance capacitor (RC) circuit, in response to thesecond signal and the adjusted first signal to obtain an integratedresult; and obtaining a time difference according to the integratedresult and taking the time difference as the delay difference.
 14. Themethod according to claim 13, wherein the time difference is obtainedaccording to a following formula:t=−RCln(1−v/V) wherein t is the time difference, v is the integratedresult, V is a power voltage of the RC integrating circuit, R is thevalue of the resistance of the RC integrating circuit, C is the value ofthe capacitor of the RC integrating circuit.
 15. The method according toclaim 11, wherein said process of determining control informationaccording to the delay difference comprises: storing, by the delaydifference adjustment controlling device, last two or more values of thedelay difference; and determining, by the delay difference adjustmentcontrolling device, the control information through comparing the two ormore than two values of the delay difference.
 16. The method accordingto claim 15, wherein said process of storing the last two or more valuesof the delay difference comprises: storing the last three values of thedelay difference; and the process of determining the control informationthrough comparing the two or more values of the delay differencecomprises: if the current value is larger than the last value and thelast value is larger than the second to last value, determining that thecontrol information being switching from adjusting the first signal toadjusting the second signal; if the current value is larger than thelast value and the last value is smaller than the second to last value,determining that the control information being maintaining a currentdelay adjustment value; and if the current value is smaller than thelast value, determining that the control information being increasingdelay adjustment value.
 17. The method according to claim 16, whereinthe delay adjustment value is an integral multiple of a preset delayadjustment unit; and the process of increasing the delay adjustmentvalue comprises: adding one delay adjustment unit to the current delayadjustment value.
 18. The method according to claim 11, wherein thedifferential signals comprise: test differential signals and actualdifferential signals; if detecting that the delay difference is largerthan a preset value, transmitting the test differential signals to thedelay difference adjustment device for adjustment; and if detecting thatthe delay difference is equal to or smaller than the preset value,transmitting the actual differential signals to the delay differenceadjustment device for adjustment.
 19. The method according to claim 18,wherein the test differential signals comprise: a first sequence,comprising N ones; and a second sequence, comprising N zeros; wherein Nis a positive integer.